The present invention relates to wavemeters for determining a wavelength of an optical beam.
Etalons are interference elements for the interference of light. To interfere light an etalon is arranged in an optical beam of the light to be measured or in a part of this beam and the etalon generates an optical signal, whose optical power depends on the wavelength of the optical beam to be measured.
Etalons show a dependency of the generated optical power from the wavelength of the incident optical beam. Generally, an etalon shows a dependency of the generated optical power versus wavelength, which can be described by the use of the Airy formulas. After calibration, the optical power can be measured by use of a photo detector and the wavelength corresponding to the measured optical power is determined.
A stepped etalon providing a transition between the stepped areas is disclosed in EP-A-1081474. A variable wavelength light filter based on an etalon structure is known from EP-A442738.
Basically, in optical communication networks information is transmitted by use of light of a light source (transmitter), an optical fiber and an optical receiver. Typical wavelengths used for optical communication are wavelengths in the range of 850 to 1650 nm, and particularly laser diodes with a wavelength in the range of 850 nm, 1300 nm and 1550 nm are used as light sources.
In wavelength-division-multiplexing (WDM) optical communication systems, information is transmitted simultaneously by a set of laser sources, each generating coherent light with a different wavelength (optical communication channels). Since the bandwidth of opto-electronic transmitters and receivers is limited, narrow channel spacing (typically 1.6 nm) is needed to increase the transmission capacity by using a multiplicity of communication channels. Particularly, in WDM systems there is a need to adjust the wavelength of each laser source very precisely to avoid channel interferences at narrow channel spacing etc.
To adjust the wavelength of the signals of a laser source, it is known to use an expensive and very precisely measuring wavemeter comprising a well-adjusted and complex mechanical arrangement. The wavelength of the signals of the laser source is measured, compared with a desired value by a controller, such as a PC, and the wavelength of the signals of the laser source is automatically adjusted to the desired wavelength.
WO 95/02171 discloses a Fourier-transform spectrometer that contains a birefringent optical component, removing the need for a Michelson interferometer used in conventional instruments. A suitable birefringent element, such as a Wollaston prism, is used to introduce a path difference between two light polarizations. Use of an extended light source so that all areas of the birefringent component are illuminated simultaneously ensures that different positions on the birefringent component correspond to different path differences between the two polarizations. A Fourier-transform of the resulting interferogram at the detector results in the spectral distribution of the input light being obtained. The use of an extended light source permits a Fourier-transform spectrometer with no moving parts to be achieved.
P. Juncar et al: “A new method for frequency calibration and control of a laser”, OPTICS COMMUNICATIONS, Vol. 14, No. 4, August 1975, Amsterdam NL, pages 438-441, XP002041763 discloses a method for high-precision measurement of the wave number of monochromatic radiation emitted by a single mode tunable laser. The described apparatus allows a direct measurement of the wave number, and serves as a reference for the stabilization and piloting of the laser frequency.
WO 95/20144 discloses an optical wavelength sensor which consists of a wedge shaped Fabry Perot etalon which exhibits resonance for different optical wavelengths across its width, and an array of detectors that detects the spatial disposition of resonant peaks which occur in the etalon, for comparison with stored peak patterns in a processor, so as to determine the spectral content of the incident light from an optical fiber.
WO 95/10759 discloses a spectral wavelength discrimination system and method that allow the wavelength of a beam of radiation to be accurately determined. The System comprises an optical System for gathering and directing received radiation; a wavelength selective beam-splitter, termed a Linear Wavelength Filter, for directing predetermined fractions of the beam at each wavelength into each of two output beams; a detector for receiving each output beam to sense the intensity of each output beam; and a computer for determining the wavelength of the received radiation. Intensity measurement of the output beams and selected system parameters, including the beamsplitter spectral characteristics and detector sensitivity characteristics are used in a special algorithm for performing Fourier based wavelength-dispersive analysis. The unique solution of the Fourier based -analysis is the wavelength of the beam of radiation.
U.S. Pat. No. 6,043,883 discloses a wavemeter comprising an optical component, which generates an optical beam with an optical power which depends periodic on the wavelength of the incident beam to be measured. This known wavemeter is provided with a second measurement channel, whose periodic signals are shifted by pi/2 relative to the periodic signals in first measurement channel. The first and second measurement channels either comprises a different etalon or the wavemeter comprises a single retardation plate to obtain the desired shift of the signals. After calibration of the wavemeter, the optical power of the beam generated by the optical component is measured, the measured value of the optical power is compared with the power values of the calibration data and the wavelength in the calibration data corresponding to the measured value of the optical power is determined.